The disclosed technique relates to energy conversion in general, and to methods and systems for increasing the efficiency of photovoltaic cells, in particular.
The solar radiation which reaches the Earth, is used to produce electrical power. Methods and systems of converting the solar radiation to electrical power are known in the art, such as heliothermal, heliochemical, helioelectrical, and the like.
In heliothermal processes the solar radiation is absorbed and converted to heat, which can be used for many purposes, such as house heating by warm air or water, cooling by absorption refrigeration, cooking, generating electricity by vapor cycles, and the like. In heliochemical processes the shorter wavelengths can cause chemical reactions, sustain growth of plants and animals, convert carbon dioxide to oxygen by photosynthesis, cause degradation of fabrics, plastics and paint, and the like. In helioelectrical processes part of the solar energy can be converted directly into electricity by photovoltaic cells.
A photovoltaic cell consists of a p-n junction formed in a wafer of monocrystalline material, such as silicon. The junction is formed parallel to the upper surface of the cell and this upper surface receives the incident radiation and produces current flow across the p-n junction. An array of these photovoltaic cells are formed on the wafer, coupled together in series, parallel, or a combination thereof, and the output leads are coupled across a load.
The material which photovoltaic cell is made of and the structure of the photovoltaic cell, determine an energy-gap, which characterizes the photovoltaic cell. This energy-gap, divided by the charge of an electron, defines the photovoltaic cell voltage, at which the photovoltaic cell produces electrical currents. The number of incident photons which are each of an energy, at least of the energy-gap, is proportional to the electrical current which is generated by the photovoltaic cell, at any given time. A photon whose energy is equal to or greater than the energy-gap, shall cause the photovoltaic cell to generate a current by one electron, at the voltage of the energy-gap divided by the electron charge.
If the energy of the photon is greater than the energy-gap, then the photovoltaic cell generates a current at the voltage of the energy-gap and the excess energy is converted to heat, thereby increasing the temperature of the photovoltaic cell. If the energy of the incident photon is equal to the energy-gap, then the photovoltaic cell generates a current at a single electron, and no heat is generated in the process. If the energy of the photon is below the energy-gap, then this photon has no contribution to power generation by the photovoltaic cell.
The current generated by the photovoltaic cell is proportional to the radiation flux (i.e., the number of photons reaching the surface of the photovoltaic cell per unit time, wherein the energy of the photons is equal to or greater than the energy-gap). Generally, the currents produced by the photovoltaic cells in the array are not equal to one another. For example, due to shadowing effect in a satellite, some of the photovoltaic cells receive less photons than others and thus, produce a smaller current. Other effects are due to the optical elements (e.g., lenses), which concentrate the light on the array, in a non-uniform manner.
In an array which includes power generating elements, such as batteries or photovoltaic cells coupled in series, when one of these elements fails, the resistance of that element increases and the power output of the array falls. Furthermore, if the power generating elements in the array produce different currents, then the current output of the array is equal to the lowest current produced by a power generating element in the array. Methods to mitigate this deficiency of photovoltaic cells are known in the art.
U.S. Pat. No. 4,943,325 issued to Levy and entitled xe2x80x9cReflector Assemblyxe2x80x9d, is directed to a solar energy system for increasing the efficiency of a solar cell. The solar energy system includes a reflector assembly, a concentrator and a photovoltaic receiver. The concentrator has a dish-shaped, concave, parabolic configuration and concentrates the solar rays at a focal point. The photovoltaic receiver is located between the focal point and the concentrator. The reflector assembly is located on the photovoltaic receiver.
The surface of the photovoltaic receiver is generally a square. The concentrator concentrates the solar rays on the surface of the photovoltaic receiver, as a circular image. The surface area and the diameter of the circular image are greater than the surface area and the side, respectively, of the square surface of the photovoltaic receiver. The circular image includes four marginal portions, each defined by the intersections of a side of the photovoltaic receiver and an arc of the circular image. The marginal portions lie outside the photovoltaic receiver. Hence, the solar rays in the marginal portions fall outside the photovoltaic receiver and do not contribute to the production of electric energy. The photovoltaic receiver includes four corner portions, each of which is formed by the intersection of two sides of the photovoltaic receiver at a corner thereof and an arc of the circular image. These corner portions are outside the circular image and therefore, the photovoltaic receiver receives no solar ray at these corner portions.
The reflector assembly includes four reflector subassemblies. Each reflector subassembly includes a pair of reflector elements. Each pair of reflector elements is located on each marginal portion. Each reflector element originates from the intersection of the side of the photovoltaic receiver with the arc of the circular image on the marginal portion and converges with the other reflector element in the pair, thus forming an apex. The apex is located between the photovoltaic receiver and the concentrator. The reflective surface of each reflective element is bicurved and concave in two directions, such that the solar rays which would otherwise strike the marginal portions, are reflected to the corner portions.
U.S. Pat. No. 4,162,174 issued to Kaplow et al. and entitled xe2x80x9cSolar Cell Arrayxe2x80x9d, is directed to a system for increasing the electrical power generated by a photovoltaic cell. The photovoltaic cell has a square configuration, while the incident light is generally circular. The photovoltaic cell includes a plurality of solar cell segments and each solar cell segment includes a plurality of unit solar cells. The length of each solar cell segment is inversely proportional to the distance of the solar cell segment from the center of the circular image of the incident light. The unit solar cells are coupled together in series and the solar cell segments are coupled in parallel to a load.
U.S. Pat. No. 6,020,553 issued to Yogev and entitled xe2x80x9cPhotovoltaic Cell System and an Optical Structure Thereforxe2x80x9d, is directed to an optical structure for increasing the electrical power produced by a photovoltaic cell. The optical structure is a transparent three dimensional body, having a bottom surface upon which light impinges and a top surface from which the light rays emerge. An array of cell-attaching active areas is formed on the top surface, wherein each cell-attaching active area includes a non-imaging light radiation concentrator. Each individual cell of the photovoltaic cell is placed on the area portion of the respective concentrator. The geometry of each concentrator is such that the light impinging on the bottom surface, is internally reflected and emerges from the surface of the concentrator in alignment with the active portion of the respective individual cell.
U.S. Pat. No. 4,513,167 issued to Brandstetter and entitled xe2x80x9cArrays of Polarized Energy-Generating Elementsxe2x80x9d, is directed to a method of interconnecting an array of polarized energy-generating elements, such that the output of the array remains constant, when one or more energy-generating elements fail. The polarized energy-generating elements are arranged in a matrix, such that the elements in each row and column are polarized in the same direction.
According to this method, the positive pole of each element in each column is coupled with the negative pole of the adjacent element in the column. The positive pole of each second element in a column is coupled with the positive pole of the corresponding element (i.e., the element in the same row) of one of the two adjacent columns. The positive pole of each alternate second element in a column is coupled with the positive pole of the corresponding element (i.e., the element in the same row) in the other of the two adjacent columns.
U.S. Pat. No. 5,928,437 issued to Dillard and entitled xe2x80x9cMicroarray for Efficient Energy Generation for Satellitesxe2x80x9d, is directed to a microarray of photovoltaic cells for reducing the effect of shadowing in the solar power system of a satellite. The microarray includes a rear interconnect, an optional substrate, a solar cell junction and a front interconnect. The solar cell junction includes an array of small solar cells coupled together in series. The optional substrate provides structural support during manufacture. The optional substrate includes a plurality of through-holes. The rear interconnect includes a plurality of rear interconnect pads and the front interconnect includes a plurality of front interconnect pads.
The rear interconnect, the optional substrate, the solar junction and the front interconnect are assembled, such that the through-holes are aligned with the rear interconnect pads and the front interconnect pads. The through-holes provide passages for soldering the rear interconnect pads to the front interconnect pads. The rear interconnect and the front interconnect provide series and parallel electrical coupling between the individual solar cells of the solar cell junction. The microarray covers small, irregularly shaped, or non-planar surfaces of satellites.
It is an object of the disclosed technique to provide a novel method and system for converting solar energy to electricity, which overcomes the disadvantages of the prior art.
In accordance with one aspect of the disclosed technique, there is thus provided an energy converting system which includes a cell array and a light concentrating unit directing concentrated light at the cell array. The cell array includes a plurality of cells, wherein the cells are coupled together according to the flux of the concentrated light which reaches each of the cells. Thus, the cells which receive light of substantially the same flux, are coupled together. Since the output current of a group of cells is limited by the cell whose output current is the lowest, the current loss in each group of cells thus coupled together, is substantially low and the power output thereof is substantially high.
In accordance with another aspect of the disclosed technique, there is thus provided a method for coupling together a plurality of cells in a cell array. The method includes the procedure of determining a lowest one of a plurality of cell flux values of the cells, in each of a plurality of groups, of each one of a plurality of array architectures. The method further includes a procedure of determining the difference between the lowest cell flux value in each of the groups, and each of the other cell flux values in the group. The method further includes the procedures of determining the sum of the differences for each of the array architectures and selecting an array architecture having a lowest one of the sums.
In accordance with a further aspect of the disclosed technique, there is thus provided a method for coupling together a plurality of cells in a cell array. The method includes the procedure of determining the sum of a plurality of cell flux values of the cells, in each of a plurality of groups of each one of a plurality of array architectures. The method further includes the procedure of determining one of the groups in each one of the array architectures, the group having a lowest sum of the cell flux values. The method further includes the procedure of determining the difference between the lowest sum and the sums in other groups of each one of the array architectures. The method further includes the procedures of determining the sum of the differences in each of the array architectures and selecting an array architecture having a lowest sum of the differences.